Aluminum parts are widely used in the aerospace, aircraft, and automobile industries. In many cases, aluminum parts must be cleaned and etched prior to being inspected using a penetrant dye process. The current practice is to clean parts manually with a solvent wipe and then perform a chemical etching on the parts in an immersion tank. There has been a long-standing need to combine the cleaning and etching steps in a single stable tank immersion instead of in multiple steps. By using a single stable tank immersion, the cleaning and etching results will be improved, manufacturing costs will be reduced, and the amount of solvent emissions will be minimized. The term “hybrid etch” refers to the combination of cleaning and etching in a single tank.
There is a commercially-available prior art liquid cleaning and etching concentrate (described in more detail below) which performs the cleaning and etching functions well when the solution in the tank is fresh. However, as the solution ages in service, the etch rate becomes too slow and a hard insoluble scale is formed in the bottom of the tank which makes it difficult to clean the tank. The prior art concentrate contains the following components:                (a) 11–55 grams/liter of phosphoric acid;        (b) 11–55 grams/liter of hydrogen fluoride (HF) (hydrofluoric acid);        (c) 110–165 grams/liter of sulfamic acid (H3NO3S);        (d) 110–165 grams/liter of propylene glycol monomethyl ether (a solvent); and        (e) balance water.        
In commercial practice, the above concentrate is supplied in 55-gallon drums and is added to a tank which is one-half full of deionized water. Thus the concentrate is diluted on a 1:1 basis. Accordingly, the bath in the tank will contain the following constituents:                (a) 5.5–27.5 grams/liter of phosphoric acid;        (b) 5.5–27.5 grams/liter of hydrogen fluoride;        (c) 55–82.5 grams/liter of sulfamic acid;        (d) 55–82.5 grams/liter of propylene glycol monomethyl ether; and        (e) balance water.        
As described above, an aqueous solution of the prior art product performs the cleaning and etching functions well when the solution in the tank is fresh, but as the solution ages in service, the etch rate becomes too low and a hard insoluble scale is formed in the bottom of the tank which makes it difficult to clean the tank. An applicable process specification, BAC 5786 (a process specification of The Boeing Company), requires a minimum etch rate of 0.8 mils per side per hour. BAC 5786 is incorporated herein by reference.
Previous efforts by others to stabilize the etch rate were not successful. In addition, previous efforts by others to keep the scale from forming were not successful. Therefore, there is an unmet need in the art for a process for cleaning and etching that stabilizes the etch rate and that prevents scale from forming. The present invention meets this need.